The present invention relates to a method of connecting a cordless telephone by radio, particularly a cordless telephone of the type not using a control channel.
A cordless telephone is constituted by a fixed unit and a portable unit which serve as a base station and a mobile station, respectively. The portable unit is connectable to the fixed unit over a radio link. Each of the two units includes a transmitter and a receiver which permit full duplex communications. To promote effective use of electromagnetic waves, a number of such cordless telephones share a plurality of radio channels. Each telephone searches for an idle radio channel and connects its fixed and portable units to each other over the idle radio channel. For this reason, the fixed and portable units are individually constructed to tune the transmit and receive frequency to any of the radio channels by a frequency synthesizer system. Each telephone is provided with a particular identification number (ID No.) so that the fixed and portable units may identify electromagnetic waves which are sent from each other.
The radio connection systems for cordless telephones may generally be classified into two types, i.e., a system using a control channel and a system not using a control channel.
In the control channel type connection system, a fixed and a portable unit each waits on a control channel. The fixed unit received an incoming call and the portable unit on which a call is originated send respectively a calling signal including the ID No. and channel information to be used and a call origination signal. Then, the fixed and portable units tune their transmit and receive frequencies to the same radio channel. The control channel is shared by the cordless telephones on a time-sharing basis.
On the other hand, the prior art connection system which does not use a control channel causes each of the fixed and portable units in a waiting condition to sequentially tune the transmit and receive frequency to radio channels to thereby cyclically monitor the radio channels available. The fixed unit received a call or the portable unit on which a call origination request has been entered fixes its transmit and receive frequency to one of idle radio channels and then sends a calling signal including the ID No. or a call origination signal over that channel. Since the other unit is cyclically monitoring the radio channels, that unit received the calling signal including the own station's ID No. or the call origination signal fixes the transmit and receive frequency to that radio channel. Thereafter, either a call reception flow or a call origination flow is executed over the particular radio channel. With the radio connection system without a control channel, it is possible to assign the entire radio frequency band available to the radio channels for communication and, hence, efficient use of electromagnetic waves is achievable.
However, the prior art radio connection system without a control channel has a problem left unsolved, as follows. Assume that when the fixed unit transmits a calling signal to the portable unit over a certain radio channel, the portable unit is brought into an off-hook condition before it receives the calling signal. Then, the portable unit decides that a call is to be originated and sends a call origination signal to the fixed unit over another radio channel. When the fixed and portable units are tuned to different radio channels as stated, not only the radio connection for the origination of a call but also the radio connection for the reception of a call are apt to fail.